There's a reason why Google (voice) Search is the most popular.

One (voice) search engine understands what you want, much better than the others.

I know that iPhone users will insist that this is wrong, but how often do you see iPhone users using Siri to complete tasks?  And I know that Cortana users will insist that the new Microsoft ad showed that Cortana was better at understanding and executing tasks, but none of these people have used Google Now.

I use Google Now, and it's just scary at how good it is and delivering what you want.  It's not perfect by a long shot (12% according to that chart) but it's really good!